Flexcom > Data Inputs > $LOAD CASE

Purpose

To provide a link between Flexcom and FAST's wind turbine control module ServoDyn in a wind turbine simulation.

Theory

Refer to Coupling between Flexcom and ServoDyn for further information on this feature.

Keyword Format

A block of lines which provides the key pieces of information to couple ServoDyn to Flexcom:

TURBINE SET=Turbine Element Set Name

ROTOR INERTIA=Rotor Inertia

GEARBOX RATIO=Gearbox Ratio

INPUT FILE=Path to ServoDyn input data file (.dat)

[FLEXCOM MANUAL OVERRIDE=Start Time, End Time, Min Blade Pitch, Max Blade Pitch, Blade Stiffness]

Table Input

Input:	Description
Turbine Set:	The portion (set of elements) of the finite element model which represents the turbine assembly which will be yawed by the yaw controller. See Note (a)
Rotor Inertia:	Rotational inertia of the rotors about the low-speed shaft which is , the integral of the rotor (blades + hub + low-speed shaft) mass by the distance from the axis of rotation.
Gearbox Ratio:	The ratio of the high-speed to low-speed shaft speed. This value should be greater than zero and equal to unity for a direct-drive turbine.
Input File:	The path to the ServoDyn control data file. See the Turbine Component for additional details.
Flexcom Manual Override Start Time:	Time to start the manual override feature which gives you some further control over the model behaviour during the initial transient phase.
Flexcom Manual Override End Time:	Time to end the manual override feature.
Manual Override Min Blade Pitch Angle:	The minimum blade pitch angle.
Manual Override Max Blade Pitch Angle:	The maximum blade pitch angle.
Manual Override Blade Stiffness:	The bending stiffness of the blade.

Notes

(a)The turbine set should contain all elements contained in the nacelle assembly that are to be yawed about the tower vertical axis.

(b)All four entries relating to Flexcom override feature are optional and by default no override is modelled. If either of the pitch entries is specified, then pitch control override is enabled, and each parameter is assigned a suitable default value if not explicitly specified (minimum = -90 degrees, maximum = +120 degrees). If a blade stiffness value is specified, it applies to both the flapwise and edgewise directions. During the initial transient phase before the rotor has reached steady-state conditions, increasing the blade pitch angle or blade flexural stiffness may be helpful in preventing rotor overspeed or tower strike. This is particularly relevant to the flexible blade model in which the initial rotor speed is 0rpm by definition as the rotor starts from a stationary position in the Flexcom simulation. As the rotor accelerates, the blade pitch angles provided by ServoDyn may not be sufficiently large to reduce the acceleration, resulting in significant rotor overspeed. Although temporary, the overspeed can result in wind forces and blade deflections which are unrealistically large, possibly leading to tower strike and simulation failure. Tower strike is not an issue for the rigid blade model, but large rotor speeds can trigger a critical Mach number error within the aerodynamic solver, again leading to premature simulation failure.

(c)Note also that ServoDyn includes a manual pitch override feature – which is more complex and powerful than the Flexcom manual override feature – but you would need to edit the ServoDyn input file to avail of this option. ServoDyn input files are text based and created manually outside of Flexcom. As you will typically consider a range of wind speeds, you could potentially require several ServoDyn input files, so you may find it easier to use the Flexcom override feature instead, which you can control from within the keyword file and parameterise with respect to wind speed if necessary.